Just so you are aware, a single 690 4gb is almost the same as two 680 2gb's in SLI, only they are on one PCB and down clocked.
If you go with 2 680's or a single 690, you will have to go with lowered settings and no AA on many if not most games in 3D, which may not be so bad. Generally speaking 3D doesn't need as much AA to look good.
Yes, I've read that the 4GB 680 might make a different with the higher settings. Also knowing two 680's is basically a 690, I figured three 4GB 680's would be able to handle max settings with today's games where dual 690's might not.
My guess is four GTX 670's might not be worth it though.